Salmonellae infections continue to be a worldwide health problem with approximately 1 million cases of gastroenteritis in developed countries and more than 10 million cases of typhoid fever in developing countries each year. Invasive Salmonella species enter a host following ingestion of contaminated food or water. The organisms move into the small intestine where they specifically invade M cells of Peyer's patches within the small intestine. These bacteria possess the ability to enter mammalian cells by a mechanism which appears to be unique. Internalization occurs following bacterial binding to the surface of a cell, stimulation of actin polymerization, and membrane ruffling. The ability of Salmonella to invade host cells is regulated by growth conditions. High-oxygen growth conditions repress the invasive phenotype and low-oxygen growth induces the ability of the bacteria to invade. Recently, several oxygen-regulated lacZ fusion strains which had lost the ability to enter mammalian cells were identified. It is likely that the products of some, if not all, of these genes are directly involved in stimulating the unidentified host signal transduction pathway in nonphagocytic cells which allows uptake of pathogenic Salmonella species. These genes will be cloned from an S. typhimurium gene bank by complementing the invasion defect of the noninvasive mutants. Following isolation of the genes, the DNA sequence of each gene will be determined and analyzed. Defined noninvasive S. typhimurium mutants will be constructed and used to determine the importance of each gene in M cell invasion and mouse virulence. Other work will concentrate on defining the function of the bacterial invasion proteins. Invasion proteins will be purified from gene fusion vectors and the purified proteins will be used to raise rabbit antibody. The anti-invasion protein antibodies will be used in studies to determine the location of the Salmonella invasion proteins during the entry process, as well as to determine whether the ability of the bacteria to invade can be blocked by preincubation with specific antibody. Another approach to study interactions between S. typhimurium and host cells will use a newly described genetic assay to functionally screen for protein-protein interactions between Salmonella invasion proteins and host cell proteins. This work will contribute to the present understanding of the Salmonella invasion mechanism. It is also likely that the study of the interactions between S. typhimurium and host cells will provide as much information about the biology of cells as it will about the bacterial pathogenic mechanisms.